Engine Performance problem
1992 Pontiac Sunbird 6 cyl Front Wheel Drive Automatic 115000 miles
I bought a 92 pontiac sunbird with the 3.1 mutliport engine. Since I bought it I've completed regular maintenance such as oil change and tune up. The car ran fine before the tune up just started real hard. However, since i've done the tune up the car seems like it's been backfiring (didn't do this before tuneup) going 40mph, 50, or 60 mph. If I hit the gas, the car goes through all these speeds and runs fine. If i'm going down a road at exactly 40 mph the car starts shaking and stuttering. I have not gotten a check engine light yet, and do not own a scan tool to see if there are any codes. Seeing how I don't know when any of the sensors were changed I was planning on changing the IAC, the MAP, and the o2 sensor. Please let me know what you think it may be and if I should change those sensors too. Thank you for your time and I love the site.